Color imaging in solid state or digital video cameras is typically performed with different types of semiconductor-based imagers, such as charge coupled devices (CCDs), complementary metal oxide semiconductor (CMOS) photodiode arrays, charge injection devices and hybrid focal plane arrays, among others.
A CMOS imager circuit generally includes an array of micro lenses, color filters and a photo imager which converts a color filtered light signal into a digital form using a read out array and digital processing. The photo imager portion of the imager includes a focal plane array of pixels, each one of the pixels including either a light sensitive area such as a photogate, photoconductor or a photodiode overlying a doped region of a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel and includes at least an output field effect transistor formed in the substrate and a charge transfer section formed on the substrate adjacent the photogate, photoconductor or photodiode having a sensing node, typically a floating diffusion node, connected to the gate of an output transistor. A device layer above or surrounding the photo sensitive regions contains wire connections to the photodiodes and some or all of the elements of a read-out circuit among other things. The imager may include at least one electronic device such as a transistor for transferring charge from the charge accumulation region of the substrate to the floating diffusion node and one device, also typically a transistor, for resetting the node to a predetermined charge level prior to charge transference.
Color imaging photodiode systems suffer from a variety of problems. For example, light intensity losses at the photo sensitive areas area due to absorption or diffraction occur as light enters the micro lenses, passes through a color filter as well as intervening layers until incident light passes into the light sensitive area of a photodiode or photogate.
Optical distortion causing, among other things, light intensity losses result from a number of design factors. Pixel surfaces above a photodiode are constructed to include, for example, color filters for red, green, blue, or cyan, magenta or yellow, depending on technology used, which are delineated on a flat planar surface. The light received by a photodiode is influenced by the materials and depths of a substrate above the photodiode. The intensity of light which reaches a photodiode which is underneath a stack of layers on an imager device is dependent on the wavelength of the light which is transmitted through color filters and or substrates due to thin film interference effects and index of refraction changes based on the depths and materials used in the imager's substrate.
Some designs vary the distance of the photodiode from the top surface of the imager in an effort to adjust for the effects of refraction and absorption within the substrate above a photodiode. Varying the photodiode distance from the top surface of the imager greatly increases the cost of imager manufacturing. Such complexity adds to design costs and does not adequately address design limitations on the ultimate transmission of photons to the light sensitive area of a photodiode or adequately increase the maximum photon intensity which can be captured by a given photodiode. Thus, a new approach is needed which can enable simplified photodiode construction while still improving or optimizing photon transmission to various photodiodes which receive different wavelengths of light, e.g., blue, red or green.